In semiconductor packaging, care must be taken to insure that heat generated by a semiconductor die can be adequately dissipated. Commonly, the die of a semiconductor device is mated with a heat spreader to conduct heat away from the die. Typically, the heat spreader operates as a heat sink to dissipate heat from the die. In some implementations, a finned heat sink is cemented to the heat spreader to enhance the heat dissipation properties of the heat spreader.
FIG. 1 illustrates a commonly employed structure for a semiconductor package design. The depicted illustration is a cross-section view of a commonly employed implementation of a conventional semiconductor package 100. In the depicted structure, a semiconductor die 102 is mounted with a packaging substrate 101. Commonly, solder bumps 104 are subject to a reflow process to physically attach and electrically connect the die 102 with the substrate 101. Additionally, the die 102 is further secured to the substrate 101 by underfilling the die 102 with an encapsulant material 103.
Additionally, the package is commonly stiffened by the attachment (or formation of) of a stiffener 106 to the substrate 101. Frequently, the stiffener is attached using an adhesive 107. Such adhesives commonly comprise epoxies, but may also consist of numerous other adhesive materials that are commonly used for such purposes. Such materials are known to persons having ordinary skill in the art. Additionally, in some approaches, a stiffener 106 can formed by depositing stiffener materials (e.g., metals) directly on the substrate 101. Generally, although not exclusively, the stiffener 106 runs around the entire edge of the substrate 101 encircling the entire die 102.
A heat spreader 110 is then attached to the “top” of the package. Commonly, the heat spreader 110 is secured to the package using another layer of adhesive 107′ on top of the stiffener 106. The adhesive 107′ affixes the heat spreader 110 to the stiffener 106. Importantly, thermal contact between the heat spreader 110 and the die 102 is facilitated by the presence of a layer 108 of heat conducting thermal transfer material. Typically, such thermal transfer material has been, until recently, comprised of a resin based material with a conductive filler. Examples include organic resins having metal fillers. However, with increasing processor speed (and therefore increasing die temperatures) come a need for increased cooling efficiency for die packages. In general, this means that it is desirable to improve the heat conduction between the die 102 and the heat spreader 110. Although suitable for many implementations, the present resin based thermal transfer materials can be improved.
In one approach, designers have sought to replace the present resin based thermal transfer materials with solder materials. Such solder materials have better thermal conductivity and so in some ways are a superior choice. However, such solder layers have their own limitations. For example, in many implementations the space between the die 102 and the heat spreader 110 can be in the range of 30 to 250 microns (μm). It has been discovered that current techniques of solder layer formation have difficulties reliably and uniformly depositing or coating surfaces to form a solder layer of such thickness. Some techniques can require that many layers of solder are formed, one over the other in order to establish solder layers of appropriate thickness. Additionally, solder layers are adhered to the heat spreader 110 and the top of the die 102 using reflow processes. If a thick solder layer 108 is used, the reflow process can cause pooling of the solder to create unevenness in the solder layer. This unevenness can prevent good thermal contact between the heat spreader 110 and the die 102. Additionally, these excessively thick solder layers can overflow and excess solder can be “squeezed” out of the space between the die 102 and the heat spreader 110. Such excess solder can overflow onto conductors or cause heat related damage to portions of the package 100. Thus, for these and other reasons, improved structures and methodologies for establishing good thermal transfer between a die and heat spreader are desired. The principles of the present invention are directed toward methods and apparatus for constructing and implementing improved multi-layer heat transfer elements.